1. Field of the Invention
This invention relates to an apparatus for suppressing the vibration of an optical part used in the exposing apparatus of an image reader or an image forming apparatus.
2. Description of Related Art
FIG. 6A of the accompanying drawings shows a cross-sectional view of an image reader.
As a method of reading an image, in the recent digital copying machine, an original image inputted from an optical scanning system comprised of a mirror or the like is converted into an electrical signal by a reading element 150 comprising a charge coupled device (CCD), and image data is sent to an image forming apparatus.
The data sent to the image forming apparatus schematically shown in FIG. 6B of the accompanying drawings becomes a laser beam applied from a semiconductor laser source to a rotary polygon mirror 151 as a laser and forming image data in the main scanning direction, and this laser beam is reflected by a turnback mirror 153 via an imaging lens, and the surface of a photosensitive drum 154 is exposed to the laser beam, whereby a latent image is formed thereon.
By such a process, image data is formed on the surface of the photosensitive drum1154, but in an image reading apparatus, an optical system generically named a mirror table and an optical mirror are scanned in the sub-scanning direction to read the original image.
The optical mirror in the image reading apparatus is comprised of three mirrors called a first mirror 145, a second mirror 147 and a third mirror 148, the first mirror 145 being installed on a first mirror table 144, and the second mirror 147 and the third mirror 148 comprising two optical scanning systems installed on a second mirror table 146.
Also, the second mirror table 146 is moved at 1:2 relative to the amount of movement of the first mirror table 144 to keep the optical path length constant, but at this time, the optical mirror itself has its opposite ends fixed to the first mirror table 144 and the second mirror table 146 by leaf springs and thus, the mirror is in a state in which the opposite ends thereof are supported, and tends to scan chiefly in an outward direction relative to the reflecting surface of the mirror by an inertial force due to scanning or the vibration of a driving motor or a driving belt or the like for driving the optical scanning systems while maintaining a vibrating state called a bending mode.
Thereby the optical axis is read while being shaken relative to the main scanning direction of the surface of the CCD 150, thus causing the bad quality of image attributable thereto.
Also, in the image forming apparatus, the turnback mirror 153 is vibrated by the rotational vibration of the rotary polygon mirror 151 or disturbing vibration or the like transmitted from other place, and the laser beam applied from the rotary polygon mirror 151 to the turnback mirror 153 causes the surface of the photosensitive drum 154 to be exposed thereto while causing the bad quality of image called “uneven pitch” on the surface of the photosensitive drum 154 by the vibration of the turnback mirror 153, thereby forming a latent image.
Such an image reading apparatus and such an image forming apparatus need be made higher and higher in the image reading speed and the image forming speed for the higher speed in recent years, and in order to meet the requirement for the higher accuracy of image, it has become necessary to reduce the vibration of the optical axis, that is, to minimize the amount of vibration of the mirror.
Heretofore, in order to meet such a requirement, it has often been practiced to stick an anti-vibration member such as a metal plate directly on the mirror, or to adopt a countermeasure for vibration as by changing the method of fixing the mirror, and the technique disclosed in Japanese Patent Application Laid-Open No. 8-106129 proposes to stick a reinforcing member on the back side of the mirror to thereby obtain a vibration suppressing effect, and the technique disclosed in Japanese Patent Application Laid-Open No. 5-88097 proposes an optical axis detecting method in an image forming apparatus and exposure position control by mirror reflection angle control.
In the case of the above-described prior art, however, the following problems have arisen.
When as in the technique disclosed in Japanese Patent Application Laid-Open No. 8-106129, an attempt is made to obtain the vibration suppressing effect by glass or the like in the mirror surface, planarity relative to the mirror surface can be secured to some extent, but the unstable behavior of the optical scanning system by a change in the centroidal position of the mirror table itself, and the fluctuation of motor torque during the optical scanning in the image reading apparatus by the addition of mass may be caused by the mass of the mirror.
Also, as one of the vibration suppressing effects by the sticking of an anti-vibration member such as a reinforcing member, it is intended in almost all cases to let the natural vibration of the mirror itself by increasing the rigidity of the mirror escape the high frequency side and avoid the resonance with the driving frequency of the mirror table or the optical motor and thus, the reinforcement of the rigidity of the mirror is necessarily effected on the mirror in a wide range. Therefore, the mass as the anti-vibration member becomes great and at the same time, when this anti-vibration member is a metal plate or the like, the planarity of the mirror may be remarkably spoiled and at the same time, these countermeasures for vibration are often effective only for respective objects and are short of universality for many types of machines.
Also, it may result in the complication of structure by the installation or the like of a mirror rotary mechanism and lead to an increase in cost to supplement the beam reflected during the observation of the positional deviation of the optical axis as in the technique disclosed in Japanese Patent Application Laid-Open No. 5-88097.
Moreover, the control of the irradiating position is done by rotating the mirror, and this is not effective for the bending vibration of the mirror which occurs frequently in the optical scanning system.